why would it be ? dont listen to everything your mechanic says
he might have also suggested to make the cooling fans direct and remove the thermostat valve because "woh thandy elakon k liye hy"
Synthetic oils are better at resisting heat then mineral oils
Toyota has recommended 5w30 semi or fully synthetic oil for your car

For vitz, Havoline synthetic is the only and best. I have been running vitz 1500cc on same for more than 30,000kms. Enhances fuel economy and runs 5000km on every change.

yes because the oil recommendations comes from manufacturer.
Example. The recommended oil for 1ZR-FE & 2NZ-FE is 5w-30.
Whereas its 0w-20 for 1KR-FE and 15w-40 for 2C. While all these engines belongs to Toyota.